Understanding Schematics

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From Schematic to RealityUnderstanding Schematics

Schematics are the lingua franca of electronics. They provide a concise and comprehensivediagrammatic description of a circuit. Plus, they are mostly standardized so once you learn thegeneral idioms of a schematic, you can decipher almost any schematic.

Schematics are especially important to stompbox building, because so many schematics areavailable. Of course, the most popular designs are represented well with PCB layouts, perfboardlayouts, vero-board, etc. But if you want to enjoy the true wealth and diversity of designs, youllwant to understand how to read schematics.

This article describes schematics, their symbols, layout and tips and tricks for reading them. Fromthere, well work on how to translate schematics into the real world in the form of things you buildon a breadboard, point-to-point, or some type of perfboard media.

Behold, The Schematic

As a starting point, lets look at a schematic of a very s imple boost pedal based on the Electro-Harmonix LPB-1.

Figure 1.1: A Schematic

You can see that there are various bits represented by symbols, all connected in various ways.Lets look at some of the big picture concepts:

Left to Right: The first thing to notice is that you read the schematic left-to-right: the inputon the left feeds the signal through parts and pathways in the middle to an output on theright. This left-to-right convention is not universal, but it is probably the most commonlayout for a schematic.

Power and Ground: The top area of the schematic shows some type of power (in ourcase, 9 volts Direct Current, the same thing that comes out of a 9 volt battery). The

bottom of the schematic shows grounds. This directly maps to the physical arrangementof our power source, again, a 9 volt battery. The top of the schematic is showing thepositive (+) voltage, and ground represents the negative (-) side.

Symbols: Components are denoted by a standardized set of symbols, each representing aspecific type of component. For example: a resistor:

Each symbol shows a part number and a part value or type. R1 denotes two things.First, the R signifies a resistor. Even though the schematic symbol itself is unique toa resistor, it is helpful to denote the part type. This is also a somewhat standardizedformat: R for resistor, C for capacitor, Q for transistor, VR for variable resistor, etc.The number part is just a sequential counter that makes it easy to cross reference

against a parts list. The number also makes it easier to talk about schematics. (Its alot easier to say change the R1 value to 500K for more bass than to say changethe first resistor that is connected from the input to the ground, before the firstcapacitor, for more bass.)

Connections: The connections between components are shown by lines. That is easyenoughanywhere there is a line, you are reading that there is a conductor (a wire or the

Understanding Schematics 15-10-2012

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copper trace on a PCB). Where the connectors cross over can be kind of tricky becausethere is no real standardized way of showing it. Is it just crossing over with no connection,or is it connected? The following diagram shows the three most commonly used connectionrepresentations:

Figure 1.2: Various Ways of Depicting Connected Lines

In the first example on the left, a dot shows interconnecting lines. So A, B, C and F are allconnected together. Lines that pass over another line are not connected, so D is only connectedto E. In the second example, dots are not used. Instead, a line that intersects without the little

hump pass over, is connected. So the first and second diagrams are the same. The thirdexample shows another where the dot signifies a connection, and non-connected crossing linesdo not use the hump pass over convention.

Inputs and Outputs

For stompbox designs, you almost always have an input and an output. Unfortunately, how

these inputs and outputs are represented on schematics is all over the place. In the moststandard form, some of the details about input and outputs are left off schematics becausethese details remain standard across stompboxes.

So when you look at a schematic like this, you are dealing with a sort of shorthand that theschematic author used.

Figure 2.1: Shorthand Depiction of Inputs and Outputs

If you look at the input side of the schematic, it is one wire. But the plug on the end of your

guitar cable has two connectors. WTF? This is an example of shorthand, and heres how theschematic maps to the real world.

Figure 2.2: Mapping Shorthand to the Real World

The tip of the plug always carries the signal, and the sleeve of the plug is always connected toground. So when you see the simplified form, it is assuming you will connect to tips of your plugsand jacks to input and output, and both sleeves will be connected to ground.

There are other ways of representing inputs and outputs on schematics. For example:

Figure 2.3: Another Way to Show Inputs and Outputs

In this example, a more literal form of schematic symbol is used for the input and outputs. Itshows the jack part connected to ground. So Figure 2.3 is electrically identical to Figure 2.1.

Power

Your stompbox circuits will mostly use a very simple power scheme: a battery or AC/DC adaptorthat provides a positive voltage and a negative voltage. The positive side of your power supplygoes to the part of the schematic that shows power input, and the negative side goes to ground.In the case of bi-polar supplies, that is not the case, but such a supply is not that common so




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we cover that separately.

Referring to our simplified schematic form again:

Figure 3.1: Power Representation

You can see that the positive side of the battery is represented by a symbol denoting + voltage.The negative side of the battery is ground. There are other forms you will see in schematics,such as when batteries are actually shown as a schematic symbol.

Figure 3.2: Battery on the Schematic

So as with other forms of shorthand, Figures 3.1 and 3.2 are electrically identical. One of thedrawbacks on Figure 3.2 is that it is showing a battery, whereas you may want to connect yourcircuit to a battery and an AC/DC adaptor. A small point to be sure, but it illustrates anotherexample where schematic shorthand can be useful.

To round out this discussion of power and input/output shorthand, heres the bo0ster schematicre-drawn to show grounds in the non-shorthand way:

Figure 3.3: The Revised Booster Schematic

Switching

Another confusing aspect can be the switching arrangement. For example, when you look at theschematic in Figure 1.1, there is no on/off switch for the power, nor is there any switching forbypassing the effects. As with input and outputs, the design of power switching and bypass

switching is usually assumed. In other words, we assume that when we build an actual pedalfrom the schematic, we will use the standard 9 volt battery clip wired to the standard 2.1mm DC

jack, all in a standard way.

Because this power scheme hardly ever changes, there is no real reason to repeat it on eachand every schematic. Similarly with bypass switching: the ubiquity of 3PDT true-bypass switchingis such that it doesnt make sense to draw it out in every schematic.

So how do you translate the shorthand of schematics to the real world of switching and power?Well cover that a little later when we talk about the Stompbox Harness.

Schematic Symbols

So now that we have the general lay of the land for schematics, lets delve into the mysteries ofthe symbols themselves. By and large, symbols are fairly standardized. However there are

exceptions that are introduced to cover the huge array of component types. In this section, wellcover the most commonly used symbols and point out any variations you might see.

Resistors, Potentiometers, and Trimmers

Resistors are not polarized devices, they work either way. Resistors are shown as a wavy line, likethe R3 value below.




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Figure 4.1: Resistor, Potentiometer, and Trimmer Schematic Symbols

Potentiometers have three connections, so you need to know how to match up the three

connections on a schematic with the actual pot, like this:

Figure 4.2: Matching Potentiometer Lugs to the Schematic Symbol

Trimmers, as shown in TR1 above are potentiometers also, but they are usually small plasticdevices soldered to the board as a set and forget type of affair.

The identification of resistors is simple: The letter R followed by a sequential number.Potentiometers are often denoted as VR for variable resistor but may also show up as R. Itseasy to spot the difference just by looking at the schematic symbol.

Additionally, potentiometer values are shown using standard code. Potentiometers have verysimple codes: a Letter and a Value. The code is:

A single letter, A for audio/log, B for linear

A Numeric value, i.e. 10K

So a 100k linear taper would be B100K. A 1k audio taper would be A1K. Finally, potentiometers

and sometimes trimmers) will have an additional label that denotes their function. So in Figure 4.1we can see that the VR1 potentiometer controls the volume.

Capacitors

Capacitors appear on schematics using one of two basic symbols: parallel lines or a straight lineand a curved line. In the case of parallel lines, the type is unpolarized, so for our purposes thatwill mean ceramic or f ilm capacitor. When the symbol is a straight line and a curved line, thecapacitor is polarized and the straight line side represents the positive side. Polarity may also beindicated by a + symbol.

Figure 4.3: Capacitors on Schematics

Diodes

Diodes are polarity sensitive, and the cathode side is indicated by a colored band.

Figure 4.4: Diodes on Schematics

The following graphics illustrates mapping between the schematic symbol and the actual device:




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scope of this article. However, it is useful to look at a simple example to try and weave all thethings weve learned so far back into a coherent example. So lets look at the booster schematicagain.

Figure 5.1: A Schematic

We can easily identify the input and output. The signal you want to modify is presented to theinput, the goo in the middle does the work, and presents is modified signal to the output. Letslook at each component, generally left to right. After the input jack, there is R1, a large valueresistor that connects to ground. This is something you will see very often in stompboxschematicsit helps set the input impedance of the circuit to a level where it doesnt drag theguitars pickups down to much. C1 is the input capacitor which filters and DC out of the signal. Italso controls the frequency response of the input signal as it is presented to the transistor.

R2 and R4 form a voltage divider. This simple snippet is in charge of providing half of the 9 volt

source voltage as a reference point to the base of the transistor. This reference point helps tellthe transistor how much to amplify the signal. R3 and R5 set the gain factor of the transistor,which simply means that it tells the transistors how many times to amplify the signal. The signalthen goes to C2 which removes the DC component of the signal.

Finally, we are off to the potentiometer for volume. The pot is wired as another voltage divider.Depending on where you turn the knob, you are balancing how much of the output signal goestor ground (i.e. thrown away or attenuated) and how much goes to the output. Thats itasingle transistor and a handful of components give you a nice linear boost circuit.

From Abstract to Reality: Lets Put it on a Board

One of the key reasons to learn how to read schematics is to be able to speak the language ofelectronics, the ability to look at a picture and get a general idea of what it does and how. But the

other more concrete reason is that you want to actually build something. Which leads to thecentral point of this article: how do you turn a schematic from abstract symbols to an actualworking thing?

The good news is that schematics are not all that abstract. In fact, in most cases you could layout your physical components in an arrangement pretty much the same as the schematic andthen connect wires just like in the schematic. While that makes sense, it is not really practical.There are much easier ways to do it.

On the Breadboard

Probably the easiest way to transfer the conceptual schematic to a physical dimension is to use abreadboard. Breadboards also have the advantage of non-permanenceunlike solder you canundo mistakes easily and experiment with different values. Most breadboards are convenientlyorganized in a way very conducive to stompbox hacking. Take a look at the following diagram:




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Figure 6.1: A Typical Breadboard

You can see that we have positive and negative strips running down the left and right edges ofthe boardvery convenient for connecting our various bits to power and ground. There are alsoa bunch of strips of 5. These are the places where we can insert components and wires toform a physical arrangement that maps to the schematic. (Note that the above breadboard isrepresentative of one of the most common types, but others have different arrangements.)

So, to build our LPB-1 Booster on the breadboard, we simply work through the schematic andarrange components and wire jumpers. Like this:

Figure 6.2: The LPB-1 Booster on the breadboard

As you trace through the schematic, compare it to the breadboard. Usually there is an aha!




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Printed Circuit Boards (PCBs) are probably the best way to build things if you are doing more thanone, or want a more professional result. But they require skills that are sometimes impractical forbeginners. In other words, you can do a lot more learning, testing and experimenting with theother types of reality devices discussed here. If you want to make your own PCBs, there aremany resources on the interwebz to help you. Additionally, lots of DIY sites, like General GuitarGadgets and TonePad have PCB layout artwork you can download and use.

Heres a layout for my Noisy Cricket PCB. Generally, a layout file will contain both the PCB layoutartwork itself, and a graphic showing the location and orientation of components for the board.

Figure 7.7: PCB Transfer Artwork

Figure 7.8: Parts Layout Diagram

The Stompbox Harness

Earlier, we talked about all those interesting shorthand notations found in schematics. Like thefact that true-bypass switching is usually not shown. Same for power on/off switching, thebattery connector and the power jack for an AC adaptor.

The following diagram shows a Stompbox Harness, a generalized component and wiringdiagram that forms a generic shell to place your circuit board in. It features true-bypassswitching, and dual power: either a 9 volt battery or an AC/DC adaptor.

Figure 8.1: Stompbox Harness

Note that there are several ways to accomplish true bypass wiring. Check out the following linkfrom General Guitar Gadgets for loads of information on true-bypass wiring options.

http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=33&Itemid=27

DIY Layout Creator

No discussion of creatin circuit boards would be com lete would be com lete with a nod to a fine


http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=33&Itemid=27


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fellow named Bancika. He created a free piece of software called DIY Layout Creator that is awork of genius. DIY Layout Creator allows you to graphically draw layouts for projects, using padper hole, veroboard, or printed circuit boards as your medium.

Figure 9.1: The Incredibly Cool DIY Layout Creator Software

As you can see from the above screenshot, you have a list of drag-and-drop components on theleft, a design area in the middle, and an explorer on the right. DIY Layout Creator would be coolif it was the product of a team of software engineers from a big company. But from a single guytoiling away to develop a free program, it is simply incredible.

If you are going to be doing anything of consequence in stompboxes, I highly recommend youdownload this program, give it a try, and then send a quick donation to Bancikahes a cool guy.

http://www.storm-software.co.yu/diy/index.php?project=software

Finally, there is also a huge library of layouts for you to freely download, including pad per hole,veroboard and PCB layouts at the same site.

http://www.storm-software.co.yu/diy/index.php?project=layouts

Resources

Thanks to google, the world really is at your doorstep. Here are some useful places to go as youwork with schematics, layouts, and boards.

What Where

Great gallery of layoutsincluding pad per hole,veroboard, and PCB designs

www.aronnelson.com/gallery/main.php

Bancikas DIY Layout CreatorSoftware www.storm-software.co.yu/diy/index.php?project=software

Layout Library for DIY LayoutCreator

www.storm-software.co.yu/diy/index.php?project=layouts

General Guitar Gadgets: Lotsof projects and layouts

www.generalguitargadgets.com

Runoffgroove: Lots of projectsand layouts

www.runoffgroove.com

Tonepad: Lots of projects andlayouts

www.tonepad.com

DIY Stompboxes: THE forumfor diy stompbox stuff

www.diystompboxes.com/smfforum

PCB Layout for Musical

Effects: R.G. Keen'scomprehensive book on doinglayouts right.

www.smallbearelec.com/Detail.bok?no=679

Conclusion

I hope that this short article has cleared up some of the mysteries of schematics for you. Ofcourse there are a thousand more details, variations and confusions as you start learning to read


http://www.smallbearelec.com/Detail.bok?no=679http://www.diystompboxes.com/smfforumhttp://www.tonepad.com/http://www.runoffgroove.com/http://www.generalguitargadgets.com/http://www.storm-software.co.yu/diy/index.php?project=layoutshttp://www.storm-software.co.yu/diy/index.php?project=softwarehttp://www.aronnelson.com/gallery/main.phphttp://www.storm-software.co.yu/diy/index.php?project=layoutshttp://www.storm-software.co.yu/diy/index.php?project=software


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schematics and transfer them to the real world. But hopefully you have a basic understanding ofhow they work, and how they map to the real world.

As always, I love to hear feedback, corrections, and even the occasional flame. Pop me an emailat dano/ at / beavisaudio.com

(c) 2005-2012. Some Rights Reserved - This work is licensed under a Creative Commons License

http://creativecommons.org/licenses/by-nc/3.0/

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Understanding Schematics